The present invention relates to a process for manufacturing oxirane by reacting an olefin with a peroxide compound in a liquid medium containing a solid catalyst. The invention relates in particular to the manufacture of propylene oxide or epichlorohydrin by epoxidation of propylene or allyl chloride using hydrogen peroxide.
It is known practice to manufacture propylene oxide by reaction between propylene and hydrogen peroxide in the presence of titanium silicalite as catalyst. For example, in patent application EP-A-0 659 473, such a process is performed in a reactor containing a fixed bed of the catalyst.
It is moreover known that the activity of catalysts of the titanium silicalite type in this type of manufacture falls over time. Consequently, it is necessary to separate the catalyst from the reaction medium regularly in order to be able to regenerate or replace it.
In the process disclosed in patent application EP-A-0 659 473, the catalyst in the form of a fixed bed is difficult to remove from the reactor to regenerate or replace it.
The present invention is directed towards overcoming this drawback by providing a novel process for manufacturing oxirane, in which the catalyst is easy to separate from the reaction medium. Another objective of the present invention is to provide a process which, when performed on an industrial scale, makes it possible to discharge the heat of reaction easily. This would make it possible to work at a high reaction rate, resulting in increased production efficiency.
The invention consequently relates to a process for manufacturing an oxirane in a reactor containing a liquid reaction medium, according to which an olefin is reacted, in the liquid reaction medium, with a peroxide compound in the presence of a solid catalyst and in the presence of a solvent; according to the invention, the solid catalyst is used in the form of particles and at least a portion of the particles in the reactor are in fluidized form.
One of the essential characteristics of the process according to the invention lies in the use of the catalyst in the form of particles in fluidized form. The fact that a fluid bed of particles can be used in an epoxidation reaction, in liquid medium, of an olefin with a peroxide compound in the presence of a solvent is surprising. The reason for this is that it is not established that particles of epoxidation catalyst tolerate fluidization, since these particles, by their nature, are fragile and risk being crushed or broken under the effect of the fluidization. The Applicant has now found, surprisingly, that these particles withstand fluidization without substantial loss of catalytic activity, showing little attrition and little breaking of grains. The fact that these particles can be used in fluidized form has the advantage, compared with a fixed bed, that the catalyst is easier to remove from the reactor in order to regenerate or replace it. In addition, a fluid bed regime ensures good heat exchange and thus better control of the reaction temperature and ensures a homogeneous dispersion of the catalyst in the liquid reaction medium.
The principles underlying the functioning of a fluid bed regime are described in xe2x80x9cPerry""s Chemical Engineers"" Handbook, Sixth Editionxe2x80x9d, 1984, pages 4-25, 4-26, 20-3 and from 20-58 to 20-75.
In the context of the present invention, the term xe2x80x9cfluidized formxe2x80x9d means that the catalyst particles are in continuous motion, which is not the case in a fixed bed in which the catalyst remains immobile throughout the reaction. However, the motion of the particles is limited since they remain in a zone of the reactor, referred to as the fluid bed, which is between a zone of distribution of the fluid and a zone of fall-out of the solid particles. Thus, in principle, the particles do not leave the zone of the fluid bed throughout the reaction, which is not the case in a transported bed in which the particles are entrained to all the areas of the reactor.
The fluid distribution zone contains a distributor which serves to present preferential streams of the fluid and thus to ensure a homogeneous fluid stream. The distributor generally consists of a distribution plate or a grille. The fall-out zone of the solid particles serves to stop the movement of the solid catalyst particles.
Generally, the fluidized form of the catalyst particles is ensured by a fluid which moves in the reactor from the bottom upwards so as to create an ascending stream having a rising speed such that the catalyst particles are fluidized. Preferably, this fluid is a liquid. It advantageously consists of the liquid reaction medium which contains the olefin, the peroxide compound, the solvent, which is usually water, some of the oxirane produced, and possibly by-products formed during the reaction.
Several factors contribute towards the satisfactory functioning of the fluid bed regime. Mention may be made in particular of the choice of distributor, the rising speed of the fluid, the specific weight of the catalyst particles, the diameter of the catalyst particles, the dimensions of the reactor and the height of the fluid bed. All these parameters are mutually dependent. Consequently, in order to achieve a satisfactory functioning of the fluid bed, it is necessary to select an optimum combination of parameters which can keep the catalyst in fluidized form throughout the reaction.
In the process according to the invention, any known type of suitable distributor may be used.
The rising speed of the ascending fluid is usually greater than or equal to 0.01 m/min, in particular greater than or equal to 0.05 m/min. This speed is commonly less than or equal to 10 m/min, in particular less than or equal to 5 m/min.
The catalyst particles generally have an apparent specific weight, measured by free flow in air, of greater than or equal to 0.1 g/cm3, in particular greater than or equal to 0.5 g/cm3. The apparent specific weight is usually less than or equal to 2 g/cm3 and more particularly less than or equal to 1 g/cm3.
The catalyst particles commonly have a diameter of greater than or equal to 100 xcexcm and in particular greater than or equal to 200 xcexcm. The mean diameter is generally less than or equal to 5000 xcexcm and in particular less than or equal to 2000 xcexcm.
The catalyst advantageously contains a reduced fraction of fine particles having a diameter of less than 100 xcexcm, since these fines are easily entrained out of the fluid bed and thus result in a loss of catalyst, fouling of the plant or the appearance of uncontrolled side reactions. In general, the fraction of fines is less than or equal to 5% by weight of the catalyst and in particular less than or equal to 2% by weight, for example less than or equal to 0.1% by weight.
The catalyst particles used in the process according to the invention generally contain a binder and an active element. The amount of binder is generally greater than or equal to 1% by weight of the catalyst, in particular greater than or equal to 10%. The binder content is usually less than or equal to 90% by weight of the catalyst and in particular less than or equal to 60% by weight.
The active element is generally as zeolite and preferably a titanium zeolite. The term xe2x80x9ctitanium zeolitexe2x80x9d is intended to denote a solid containing silica which has a microporous crystal structure of zeolite type and in which several silicon atoms are replaced with titanium atoms. The titanium zeolite advantageously has a crystal structure of ZSM-5, ZSM-11, ZSM-12, MCM-41 or ZSM-48 type. It may also have a crystal structure of beta zeolite type, preferably free of aluminum. Zeolites with an infrared absorption band at about 950-960 cmxe2x88x922 are suitable for use. Titanium zeolites or silicalite type are preferred. Those corresponding to the formula xTiO2(1xe2x88x92x)SiO2 in which x is from 0.001 to 0.5 and preferably from 0.001 to 0.05 give high performance. Materials of this type, known under the name TS-1, have a microporous crystalline zeolite structure similar to that of zeolite ZSM-5.
The binder generally comprises one or more silicon derivatives.
The catalyst particles may be obtained by any known means, for example by extrusion, as disclosed in patent application WO 99/28029 by the Applicant, the content of which is incorporated by reference in the present patent application, or by a spray process, as disclosed in patent application WO 99/24164 by the Applicant, the content of which is also incorporated by reference in the present patent application.